1. Field of the Invention
The present invention relates to a tow cable used for towing air or water guns behind a seismic vessel, known as a gun cable, wherein the cable comprises a reinforced air hose within a protective jacket.
2. Description of the Prior Art
Normally, a seismic vessel will tow 5 to 8 gun cables at the same time, with an array of 4 to 8 guns being attached to each cable.
A gun cable has three main functions: it
should be able to withstand a tension of up to 10 tons in the form of peak loads of short duration, and a static tension of 3 to 4 tons continuously; it
must supply the guns with compressed air (at a typical pressure of 140 kp/cm.sup.2) via at least one high pressure air hose; and it
should be able to transmit signals to the guns for actuating firing of the guns on an individual basis, and to transmit discrete signals from the guns for reporting the precise time of individual shots, as well as other electrical signals from close range hydrophones and depth sensors.
The common practice earlier was to supply air to the guns via a separate air hose that was securely lashed to the gun cable. Today, composite cables which combine all of the three main functions in a single cable are available on the market. These cables obviously provide significant advantages in the form of easier handling, but the prior art cables of this type have a number of drawbacks.
A gun cable of this type is usually constructed as follows: at
the center of the cable is one (or more) high pressure air hoses of conventional type, consisting of a nylon inner hose with polyester fiber reinforcement; outside
the air hose(s) are several layers consisting of electrical conductors and steel wire for relieving cable tension; and outermost
is a jacket of artificial material (polyurethane or neoprene, for example) for providing mechanical protection and watertight sealing.
Gun cables of the above type have been in use for about the last five years, and during this time a number of weaknesses have been uncovered. These weaknesses can be summarized as follows: the
outer jacket is not adequately watertight either because it is made of a material which itself is not sufficiently water-resistent, or more often because small holes, tears and cracks occur in the jacket as a result of normal handling under difficult conditions; the
outer jacket does not provide sufficient mechanical protection and in addition to damage to the jacket itself as mentioned above, breaks may occur in the electrical conductors if the cable is bent too sharply or if it is penetrated by sharp objects; the
air hose "works" too much inside the cable. During use, the pressure in the hose pulsates continuously, typically between 70 kp/cm.sup.2 and 140 kp/cm.sup.2. This causes the diameter of the hose to vary by up to 2 mm. There is reason to believe that the breaks occuring in the electrical conductors may in part be caused by fatigue owing to this pulsation. The outer jacket gets "blisters". Gun cables are sometimes ruined because large bulges or blisters appear in the outer jacket. This is caused by high pressure compressed air which either diffuses through the walls of the air hose or leaks out through inadequately sealed portions of the hose or at the hose termination. This air has no direct path of escape, and since the outer jackets is held in place by its own elasticity, it will blister if the jacket is airtight; the
cables have large outside diameters. The complex construction, especially owing to the large number of electrical conductors that are required, results in thick cables which are both cumbersome to handle and which exhibit high resistance to water when they are towed crosscurrent.
By far the biggest weakness of today's gun cables is their vulnerability to water penetration. Water which penetrates the outer jacket will quickly spread along the entire cable, both because of the pulsating effect of the air hose and because microscopic air pockets are formed in the cable when air diffuses from the air hose, and these pockets of air are immediately filled with water. The water results in leakage current between the conductors and causes crossing of signals, so that the cable can no longer perform its intended function. Leakage current occurs in connection with microscopic defects (pinholes) and other damage to the insulation around each individual conductor, or at the end of the cable (the termination) where the electrical conductors terminate in electrical underwater connectors.
Despite the fact that air gun cables have for several years been considered an important source of problems in connection with seismic survey of the sea bed, the cables have not been improved very much during this time. This is probably because it has proved difficult to improve single aspects of the cable without at the same time introducing new problems.
For example, in order to obtain better mechanical protection, cables have been made with the tension-relieving steel wire outside rather than inside the cable. Two full layers of steel wire cable must in that case be utilized, wrapped around the cable in opposite directions to provide torsional balance. These wires must also be of rather thick dimensions to prevent them from fraying easily. The cable one obtains as a result is thick, rigid, heavy and difficult to handle.
It has also proved difficult to find a good solution to the water sealing problem. Because all electrical insulation material is more or less hygroscopic, and because the insulation around each electrical conductor has to be thin in order not to take up too much space--which in turn increases the likelihood of point defects and wear damage--a cable in which the insulation around the conductors constitutes the sole water sealing means is not an ideal solution. The usual resource is to provide a watertight jacket surrounding all of the conductors-but in practice, this jacket says watertight for only a short period of time, and it may also create problems (air blisters) if it is too airtight.